The decline snaps a 27-month streak of year-over-year gains. The timing of this year's Labor Day weekend is blamed for some of the weakness.

U.S. monthly auto sales declined in September for the first time in more than two years, in part because of a calendar quirk that landed Labor Day weekend sales in the August report.

Automakers sold a little more than 1.1 million vehicles last month, a 4.2% decrease from September a year ago, according to Autodata Corp., an industry research firm. The single-month decline snapped a 27-month streak of year-over-year gains dating back to June 2011, according to Edmunds.com, the car shopping information company.

"We definitely felt a market pullback during the month," said Dave Zuchowski, executive vice president of national sales for Hyundai Motor America.

The calendar was one factor, he said. But politics also played a role, as consumer confidence suffered because of fears over a government shutdown and uncertainty over healthcare costs with the launch of the Affordable Care Act, Zuchowski said.

Others pinned the reversal almost entirely to the early Labor Day — the preceding Saturday, traditionally big for car sales, fell on the last day of August. This September also included fewer days for car sales than a year ago, in part because some states don't allow car sales on Sundays. (September included five Sundays but only four Saturdays.)

Analysts at Edmunds.com noted that automakers still sold more cars per selling day this September than last September.

"The American people might be questioning their faith in government right now, but they still have confidence in the economy," said Lacey Plache, Edmunds.com chief economist. "In fact, American car buyers have been remarkably resilient in the face of the Congress' antics over the past year. There's no reason to believe that auto sales will suffer in the coming days or weeks as this mess gets sorted out in Washington."

Automakers were rooting for a quick resolution to the fiscal fight in Washington.

"We have some amount of faith that the right things will be done, and will be done quickly," said Kurt McNeil, vice president of GM's U.S. sales operations.

But if the battle drags on and government employees remain out of work, it will hurt their spending power, automakers said.

In government-dependent areas, dealers reported declining consumer traffic over the weekend, said Mark McNabb, chief operating officer for Volkswagen of America, which experienced a 7.4% dip in sales last month.

"But it is certainly way too early to see what the impact is," he said.